Turnover.

Did you know that the...                                                         Image source here.

...small intestine regenerates itself every 48 hours. 
...and that the large intestine regenerates itself every 12 days.

This is why gut cancers (i.e. colon cancer) is time critical, because the cells grow so fast it is easy for the cancer cells to get out of control, fast!

However, these rapid turnover rates are normally a good thing, especially if you have digestive complaints and/or food allergies/sensitivities.  For example, if someone with a gluten intolerance eats something gluten-y by accident, then he or she will feel awful and his or her gut absorption and function will be decreased, but within two days they should be back to normal, with the gut well functioning and all. Although, depending on the amount of gluten consumed and the degree of sensitivity he or she has towards gluten, it may take the full 12 days to feel a-okay again.  But for the most part, if you eat something that disrupts the gut or have some food poisoning-like symptoms, the gut should be fully repaired as it'll be brand spakin' new within a couple of days.  Lastly, even though full month long elimination/hypoallergenic diets are best to rule-out food sensitivities, a quicker (yet much less accurate!) way of detecting a food problem would be to 100% avoid the food for a full two days. In this time the gut will repair itself and will provide a clean slate for food testing.  Then eat the potentially troublesome food and see what happens.  Nevertheless (yep, I'll say it again), this isn't the best test and a full month-long cleanse and reintroduction period is way more accurate.